Baking Models with AO
I would agree with HATMAN - Possibly inverted Normals or doubles. Go into Edit mode, Press W - Remove doubles (also available from the tool menu on the LH side). Along the bottom menu, you can also recalculate normals by going to mesh> normals>recalculate outside . They should help.

When you do these, take note of the top bar (where it says the amount of tris etc), and it will tell you if it had to remove doubles etc..